We are remodeling a recently purchased 1883 farmhouse. We have a few of the exterior walls open on the inside which I would like to air seal and waterproof while it is open. It was built with Douglas fir shiplap siding directly on the studs. The previous owner had insulation blown in about five years ago. Both siding and framing seem to be in good shape, but you can see light at some of the siding laps. I thought of rolling Prosoco R-guard on the back with tape or fabric on the joints but their technical advisors did not recommend that idea.

I feel like the house has lasted this long because there was so much air movement through the walls that any water could dry out. That protection probably goes away with insulation in the walls. Any ideas that would improve air and water sealing from the inside without causing more serious problems?

Thanks,
Andy